I currently have 154 images with a total size of 97.6MB. It worked fine with Java 11 hovering around 3.5 of 8GB when there were no players. However when I switched to Java 12 or above, the memory usage was extremely high and unstable (6/8GB), and always spiked beyond the limit (8GB) when players entered, crashing the server. I tried removing this plugin, and it worked fine.

Do you have a crash log? I just tried it on my local server using AdoptOpenJDK 15 without problems.

Also, what exact JRE are you using?

Yes, the crash log is here: https://pastebin.com/G1JkQvwQ

I'm using OpenJDK 64-Bit Server VM 16+36-alpine-r1

Hm... the crash log seems to indicates it's taking too long while BkCommonLib is doing something with the maps.

Have you tested it without BkCommonLib? Maybe there is some weird interaction between the two plugins that causes it...
Alternatively you could try increasing the server timeout value, maybe it just takes a while once.

I suspected bkcommonlib initially as well. Tried your suggestions, removed bkcommonlib, and also subsequently removed all plugins except imagemap.

With all but imagemap removed and no players inside, memory usage was still extraordinarily high and spiked unusually when it was run with java 16 (3.32/8GB); java 11 looked fine (2.2GB/8GB).

Tried both dev build and release, both yielded similar results.

Not sure if the logs below would be helpful:
With Java 16: https://pastebin.com/Kn94ucQ3
With Java 11: https://pastebin.com/mDc2n43H

Could you create a heap summary using Spark? It might show where the extra memory is going to.