Interactive fields lock after saving for first time

I've had a bit of an unexpected issue which I haven't had before. 

I created an interactive pdf doc in InDesign, with checkboxes and fillable text boxes. The doc requires multiple people to contribute and I haven't had an issue with it before. 

Recently, on a doc I was collaborating with a colleague, I've found that all interactive elements were "locked", I could select the text they had written but could not edit or write my own. I'm using the latest version of Adobe Acrobat. I had checked all permissions and security settings and everything was normal, all permissions allowed, no security etc. which is puzzling.

To test whether it was something done on their side, I took the original pdf doc I made from InDesign and opened it up in acrobat, all works as usual, all interactive blocks are working as they should, but the moment I hit the save button, all the interactive blocks suddenly grey out and "lock" in place, I can no longer touch them. I tried resaving, adding security than removing and opening again but it's firmly locked in place and no longer editable as it should be.

If anyone could share some insight or ideas into what I'm missing, that would be great.