Updated: July. 2018This is a known issue. There doesn't seem to be a fix for it as of yet. However, there is a work-around for it.
[WORKAROUND] Sims uploaded to the gallery wrongly marked as modded
https://answers.ea.com/t5/Bug-Reports/WORKAROUND-Sims-uploaded-to-the-gallery-wrongly-marked-as-modded/m-p/5145862#M21765Even if you don't have mods or cc in your game, clear the game cache and delete the the localthumbcache.package file from your Sims 4 folder to help troubleshoot the issue.
1. Save the Sim or house to the Library. It may be marked as modded.
2. Close the game, and start a new one.
3. Load the Sim or house from the library and try uploading the Sim or house again.
If you are using mods and cc in your game, move the mods folder to to the desktop, clearing game cache and delete the the localthumbcache.package file from your The Sims 4 folder and repair the game in Origin to help troubleshoot the issue.
All issues reported in the bug forum must be tested on vanilla (no mods/cc) games.
Delete Game Cache Files (What and How-To)
http://www.carls-sims-4-guide.com/forum/index.php?topic=29288.msg401662#msg401662Repair Game:
In Origin > Games Library, right-click on Sims 4 and select Repair. - Do note this repairs the game's install files for the actual game. It does not make any changes to the saved game files. Meaning that your saved games are safe, they won't be deleted. If you'd like you can copy the Sims 4 folder to a USB key.
This bug isn't actually about custom content. It's about the game thinking there is custom content in your uploads when there's not.
If the above doesn't help, please post about the issue in the thread (first link) above, letting them know what you've tried to troubleshoot the issue.
Here's a quote from Playalot that I thought was important to note:
If the house has paintings that a sim has painted using 'paint from reference' then the game auto-detects it as custom content.