This happens to me occasionally as well, and it is very annoying. If you go to jira.atlassain.com and search for "cell borders" you'll find quite a few tickets where people are complaining about borders disappearing or not rendering properly in several different situations. Unfortunately none of these tickets have a high priority, and so are unlikely to be fixed. That doesn't help you, but be assured you're not the only person with this problem.
You might want to check out some of the table add-ons here. Some are paid, some are free, and they should help you get a better table experience.
More and more people are building their careers with Atlassian, and we want you to be at the front of this wave! Important Dates Start the Certification Prep Course by 2 April 2019 Take your e...
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
You're one step closer to meeting fellow Atlassian users at your local meet up. Learn more about AUGs