I’ve tested it on versions 49 and 51 — both throw the exact same error. When I save the preset and then load it directly from the plugin, it works fine. But when I try to load the preset via a button, it fails with a 'file not found' error. I was like… WTF?

I’ll try to grab some screenshots. I always struggle with uploading pictures to forums, but I’ll give it a shot.
i find the error.
++++++++++++++++++++
Error-
your error is that the plugin call for different path, he's trying to look for the preset in: /Saves/scene/aa.json
{basically, when you call the 'Load Preset File' from a button trigger on your plugin, he's looking for the preset in: /Saves/scene/}
Solved-
i entered this path instead in the preset named (thru the button): Saves\\PluginData\\Chokaphi\\DecalMaker\\Presets\\aa.json -> and it worked.
so i guess the bug is from where your plugin call the preset, maybe you should change the presets that we save with that plugin into /Saves/scene/ instead, this will instantly remove the bug...but i'm not sure this is your wanted end results, which is understanable.
i'm asking myself, how the fuck i'm playing 3 years without your plugin, he's amazing. thanks