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 Unfortunately, I originally made a save prior to realizing that issue was occurring, and it messed things up royally, making me have to restore back up again.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.