I have not been able to confirm this locally. Can you let me know what style options, if any, you are using?

Yes, I use \documentclass[twocolumn]{aastex631}. The only package that I import outside of the class file is amsmath.

I did observe what @zpleunis saw ~2 months ago, and recorded it in my own mod of the aastex package seawander/aastex_pwned@7854f15 . Thanks for the fix, @zpleunis !

I sent an email regarding this issue on March 5 and got a response (from Greg Schwarz, actually, who I assume is the same as the above). He also found that it was only replicated when amsmath was imported, which was true in my case as well. It does seem to be a conflict relating to amsmath.

I also experienced this issue today (and using amsmath)
astro-ph actually rejects your manuscript if it includes line numbers anywhere, so might be worth fixing.

The solution in the opening posts works for me, replacing those four lines.

This is in our list for the next version. Note that you also do not have to use the \begin{acknowledgment} ... \end{acknowledgment} environment. It is primarily needed to blank out the acknowledgments when using the "anonymous" style option for dual anonymous review (DAR).

Excellent, great to see that this is being fixed.

So the alternative to the acknowledgement environment would be to just use a \vspace{} or something?

Yes, or just create a new \section*{Acknowledgment} if you really need a section header for some reason.

Just to check, is this bug been addressed? As mentioned earlier, arXiv rejects these submissions.

I also received an arXiv rejection due to this, so glad it is on the list to fix.

My workaround was to simply remove the acknowledgments environment and instead just separate that paragraph from the end of the main text body with a few \\ line breaks.

Just had an arXiv rejection today due to this.

A temporary solution is, within aastex631.cls, in lines 1240 and 1249, just comment out the \begin and \end.

A better solution would need to fix the \ifx statement at line 1239 so this doesn't execute when line numbers are turned off.

My fix: wrap both lines 1239-1241 and lines 1248-1250 with \ifnumlines .... \fi, then the acknowledgements follows the document-level line numbering setting.

Edit: also wrapping lines 1294-1297 and 1303-1306 with \ifnumlines .... \fi for the alternative spelling of acknowledgements.

ArXiv rejected our paper due to this bug. I'm glad this issue is on the list to be fixed; I suspect it affects many users. Any news on when we can expect to see a new aastex release? I appreciate the hard work that goes into this.

We are in fact beginning to scope out the next release, but dev hasn't started and given the issues, this might be big one. I'm really sorry this bit your arXiv release @matiscke! There are so many nests of dependencies that some clean out is required.