Sorry to hear that you are having Autosave problems.
Acrobat saves pending changes when you close a PDF document.
What is the version of your Acrobat app?
Do you tap the Back (<) button to properly close the document?
Where are your documents stored? On your iPad/iPhone or cloud storage (e.g. Adobe Document Cloud, Dropbox, Google Drive)?
Would you please try adding highlight to several other documents to see if changes are saved when you close and reopen them? We would like to know if the problem occurs with the particular PDF document or all of your PDF document.
Thank you for reporting the lost comments issue to us. We just released a new version (19.01.01) to the App Store and have fixed a number of issues that could be causing lost comments. Please install it and let us know if your problems have been resolved.
However, if not, in order to help us further investigate, it would be amazing if you could answer any or all of the following questions:
How long had you been commenting before you experienced data loss (in hours)?
How did you leave the Acrobat App or PDF when you experienced data loss (switching to another app, putting phone to sleep, etc.)?
Is the PDF you were commenting a cloud file (Google Drive, Document Cloud, Dropbox)? If so, which cloud service?