Definitely, especially if you don't have a recent backup. Not that you wouldn't...
I had this issue when importing my project to a new PC. When I opened the designer sometimes Custom props would be gone, and sometimes the title menu would be gone. I had to re-open the designer multiple times without saving in order to get a 'good load' then saved, and the problem hasn't seemed to come back. Unfortunately, I originally made a save prior to realizing that issue was occurring, and it messed things up royally, making me have to restore back up again. If anything, this taught me the automatic backups from ignition is a great tool to have in place in the event of any issues, I also now am in the habit of verifying when I load designer the Session props and the page configuration load appropriately prior to making/saving and changes. Never ran into this problem until I restored for the first time, however.
The release notes for version 8.1.47 appear to have a relevant entry.
Fixed an issue that caused bound custom session properties to disappear from the Designer after merging updates from the Gateway.
Ignition Release Notes for Version 8.1.47 | Inductive Automation
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My gateway is running 8.1.47 and the same problem happened today for a session custom property with the persistent option set to True.
Bad news, yesterday I've updated to 8.1.47 hoping to fix this 
Yeah we're on 47 and had it happen a few weeks ago in the office dev environment (3 devs working on it)
Last week it also happened to me with 8.1.37 (I hadn't updated the version yet), but I had the problem during some "overlapping status" caused by network errors in the office.
Today, this also happend to me with version 8.1.47.
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Had them same thing happen today running 8.1.47
Had it happen on 8.1.48 so its still with us.
Hello,
The bug is still with us 8.1.47. I noticed this happens whenever I make a CSS-related change, like a change to the stylesheet.css file. Maybe it helps, but it doesn't happen all the time. It hasn't happened for a month. When we started making CSS changes, it started happening even during the day. I'm not sure if it's related, but maybe it helps. It's very annoying and would cause major problems if it happened live.
Just chiming in to say I have had the same issue on 8.1.48. We have two devs working on a project simultaneously, but also have been editing the CSS stylesheet recently. Not sure which the issue could be related to.
Adding on, Iāve had this occur on our project with 5 devs working on it, running 8.1.48. Unfortunately unable to reliably reproduce it but weāve had it occur multiple times so far.
Had this same issue happen. Currently working on a test project in 8.1.48 (not edge, regular ignition). Launched my project one day and there were errors due to all the custom session props being gone. Just empty. The previous props consisted of 2 unbound props, and an object with several bound props under it. I was able to restore them all from a backup like @pascal.fragnoud suggested. But would hate to have this happen in production and end use.
I think what caused it was hitting the update button to āMerge any new changes on the Gateway into the open projectā.
Still happens in 8.1.48.
All session.custom props missing, not just ones with bindings.
Copy/paste did not show up with bindings, so I donāt suspect any relation to being persistent.
Did not coincide with any āRestarting gateway scriptsā¦ā log entries.
If you have multiple developers, you should communicate with each other when creating/modifying custom session properties; since itās a last-save-wins ordeal. I found with flaky connections this occurs more often. Once this is understood from all parties involved, the issue will resurface less.