I believe I have found the reason for this problem. The "factorio" and "factorio_wide" board styles are buggy. When I switch to "prosilver" board style, it works perfectly.
This support thread from the official phpBB forum might help in fixing the problem:
https://www.phpbb.com/community/viewtop ... &t=2415896
However, I believe the problem discussed in that phpBB thread is not completely identical to the problem on the Factorio forum, because they were discussing ordered lists not appearing at all, whereas the problem on the Factorio forum is that ordered lists are appearing as unordered lists.
Possibly the Factorio custom board style simply contains a small error.