Line gets bolder while animating - problem

[medaniwd]

I have a SVG with a rectangle that is build out of 3 lines. The left and right lines draw themself like they should, but the bottom one gets bolder as long as the animation is playing. 

 

1. & 2. Animation builds up - already bolder than it should be

3. There's also a slightly thicker part at the right site for a moment

4. Animation is ready and jumps to the "original" appearance - That's how the stroke-width should look like all the time

 

I'm new to it so I am trying to figure out if it's a known problem or if there's a mistake with my SVG - or something completely different

 

Thank you in advance

[OSUblake]

It's hard to say without seeing it. Please make a simplified demo showing the problem.