JamesM

Thanks, that's very helpful. I hadn't thought to rasterize a pixel layer, only vectors, text etc. But this seems to work fine for me. Hopefully it will for the topic starter, too.

Did you perhaps flip the document sometime prior to applying the Liquify filter? If you do this, Affinity Photo messes up the Liquify effect in the way that you describe. I reported the bug some time ago in this thread - https://forum.affinity.serif.com/index.php?/topic/28543-liquify-on-flipped-document/- but unfortunately it hasn't been fixed yet. Hopefully the Affinity team will do so soon, because it does affect my workflow when flipping to check and adjust a picture's symmetry and balance. In the meantime, in your case I would suggest flipping the document back again and then trying Liquify to see if it now works as expected - or open a new document and see if it works correctly on that. Personally I haven't had any trouble with Liquify in this or any build except with flipped documents.