Hey, I've faced this issue before and reported it.
LMMS doesn't behave nicely with infiniband or modifiable band EQ and compressor. Using ReaEQ will also cause unusual problems. Unfortunately, it looks like the total number of bands on your project is more than 8.
https://github.com/LMMS/lmms/issues/6223 is the issue.
There is nothing that can be done. You cannot add or modify bands because then saving breaks. Check if deleting bands works.
I found this compressor called IEM multiband, which has three bands and a visual interface and options that works more like ReaXComp than most other multiband compressors available for free. I use sometimes multiple together if i need many, but usually I've gotten used to using as few as possible (sometimes limitations make you stronger).
In any case, you can use multiple instances, and within each instance use two bands and mute the third band out. In second instance use one band, or use multiple whichever works for you. End effect is some sort of multiband compression that does the trick.
Also lmms 1.3 has a factory compressor, and iirc one of those parameters tilted the compression so it was more applied (ie higher ratio i think, maybe combined with threshold) on lower range than upper, and vice versa. I'm on my phone so i can't be sure of the abilities, haven't gotten time to touch lmms and use it properly for a few weeks now.
Even without multiband, that compressor is pretty powerful so check it out.
P.S. if you find any multiband compressor which suits your needs, and has fixed number of bands, please get back to us. I for one would love to know.
All the best with LMMS!