Assertion Failure in rts/sm/MarkWeak.c, line 375

Summary

Initially I saw the following error with various closure numbers when running tests. Seems to be related to pointers but is intermittent. Wondering if it is caused by race conditions somewhere in ForeignPtr garbage collection.

regex-parser-test: internal error: evacuate: strange closure type 0
    (GHC version 8.8.3 for x86_64_apple_darwin)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
                          
regex-parser> Test suite regex-parser-test failed
Completed 2 action(s).    
Test suite failure for package regex-parser-0.1.0.0
    regex-parser-test:  exited with: ExitFailure (-6)

However, once adding the -debug flag to GHC and running with the following, specifically file rts/sm/MarkWeak.c, line 375

regex-parser-test: internal error: ASSERTION FAILED: file rts/sm/MarkWeak.c, line 375

    (GHC version 8.8.3 for x86_64_apple_darwin)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug`

Fairly new to Haskell but in the middle of a large client project when I came across the above, so any pointers would be greatly appreciated. Been working on this for 3 days and kind of drawn a blank.

Steps to reproduce

Difficult to say. I'm working on a large application and it's tricky to track down. Any assistance knowing where to focus my attention would be much appreciated.