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?