Merging threads removes custom fields

[Spectral-Brain]

Affected version

2.3.6

Lets say I have a bug report forum and if there are 2 bugs that are the same, we merge the threads together. We want to keep the custom fields because they contain information we need, but when merging a thread, the custom fields of the thread post being merged into the other thread, no longer show and are lost.

The fields should follow the post and show instead of being removed when merging threads.