GLES3.2 Kawase Shader giving horrible artifacts - c++

Question

I am currently trying to implement a Kawase Blur shader in GLES3.2.
I found the appropriate fragment shaders on shadertoy, and implemented them as such:
Down:
#version 100
precision mediump float;
varying mediump vec2 v_texcoord; // is in 0-1
uniform sampler2D tex;
uniform float radius;
uniform vec2 halfpixel;
void main() {
vec2 uv = v_texcoord * 2.0;
vec4 sum = texture2D(tex, uv) * 4.0;
sum += texture2D(tex, uv - halfpixel.xy * radius);
sum += texture2D(tex, uv + halfpixel.xy * radius);
sum += texture2D(tex, uv + vec2(halfpixel.x, -halfpixel.y) * radius);
sum += texture2D(tex, uv - vec2(halfpixel.x, -halfpixel.y) * radius);
gl_FragColor = sum / 8.0;
}
up:
#version 100
precision mediump float;
varying mediump vec2 v_texcoord; // is in 0-1
uniform sampler2D tex;
uniform float radius;
uniform vec2 halfpixel;
void main() {
vec2 uv = v_texcoord / 2.0;
vec4 sum = texture2D(tex, uv + vec2(-halfpixel.x * 2.0, 0.0) * radius);
sum += texture2D(tex, uv + vec2(-halfpixel.x, halfpixel.y) * radius) * 2.0;
sum += texture2D(tex, uv + vec2(0.0, halfpixel.y * 2.0) * radius);
sum += texture2D(tex, uv + vec2(halfpixel.x, halfpixel.y) * radius) * 2.0;
sum += texture2D(tex, uv + vec2(halfpixel.x * 2.0, 0.0) * radius);
sum += texture2D(tex, uv + vec2(halfpixel.x, -halfpixel.y) * radius) * 2.0;
sum += texture2D(tex, uv + vec2(0.0, -halfpixel.y * 2.0) * radius);
sum += texture2D(tex, uv + vec2(-halfpixel.x, -halfpixel.y) * radius) * 2.0;
gl_FragColor = sum / 12.0;
}
The thought process is simple: the texture I want to blur is in the Primary FB, then it's passed once with a down blur to the Mirror FB, and then blurred down more and up on the Mirror FB, and finally later rendered.
The down shader works great, and produces the expected result of a small image in the top-left corner, with streaks throughout the rest of the framebuffer.
Image of shader Down
However, when trying to apply even one pass of the up shader, it starts giving horrible artifacts. Blocks of pixels blinking everywhere, and the screen is divided into 4 sections vertically, where in each one the original image gets bigger, blurrier and more glitchy.
Image of shader Up
This result is with 2 passes of the down shader and one of the up shader
The code:
const float fullVerts[] = {
1, 0, // top right
0, 0, // top left
1, 1, // bottom right
0, 1, // bottom left
};
// ...
{
auto drawPass = [&](CShader* pShader) {
glActiveTexture(GL_TEXTURE0);
if (pShader == &m_shBLUR2)
glBindTexture(PMIRRORFB->m_cTex.m_iTarget, PMIRRORFB->m_cTex.m_iTexID);
glTexParameteri(PMIRRORFB->m_cTex.m_iTarget, GL_TEXTURE_MIN_FILTER, GL_LINEAR);
glUseProgram(pShader->program);
// prep two shaders
glUniformMatrix3fv(pShader->proj, 1, GL_FALSE, glMatrix);
glUniform1f(glGetUniformLocation(pShader->program, "radius"), BLURSIZE * (a / 255.f)); // this makes the blursize change with a
if (pShader == &m_shBLUR1)
glUniform2f(glGetUniformLocation(m_shBLUR1.program, "halfpixel"), 0.5f / (m_RenderData.pMonitor->vecSize.x / 2.f), 0.5f / (m_RenderData.pMonitor->vecSize.y / 2.f));
else
glUniform2f(glGetUniformLocation(m_shBLUR2.program, "halfpixel"), 0.5f / (m_RenderData.pMonitor->vecSize.x * 2.f), 0.5f / (m_RenderData.pMonitor->vecSize.y * 2.f));
glUniform1i(pShader->tex, 0);
glVertexAttribPointer(pShader->posAttrib, 2, GL_FLOAT, GL_FALSE, 0, fullVerts);
glVertexAttribPointer(pShader->texAttrib, 2, GL_FLOAT, GL_FALSE, 0, fullVerts);
glEnableVertexAttribArray(pShader->posAttrib);
glEnableVertexAttribArray(pShader->texAttrib);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glDisableVertexAttribArray(pShader->posAttrib);
glDisableVertexAttribArray(pShader->texAttrib);
};
// draw the things.
// first draw is prim -> mirr
PMIRRORFB->bind();
clear(CColor(0,0,0,0));
glBindTexture(m_mMonitorRenderResources[m_RenderData.pMonitor].primaryFB.m_cTex.m_iTarget, m_mMonitorRenderResources[m_RenderData.pMonitor].primaryFB.m_cTex.m_iTexID);
drawPass(&m_shBLUR1);
// now draw from mirror->mirror
glBindTexture(PMIRRORFB->m_cTex.m_iTarget, PMIRRORFB->m_cTex.m_iTexID);
for (int i = 1; i < BLURPASSES; ++i) {
drawPass(&m_shBLUR1); // down
}
for (int i = BLURPASSES - 1; i >= 0; --i) {
drawPass(&m_shBLUR2); // up
}
}
What's causing the artifacts?