I also has this experience sometime, when developing new custom control.
This way, the properties has not properly set as expected.
Usually, the bug is because there is yet already saved properties in old form (your real / big project),
during I were add/remove more properties, or since I added "default" value for some property.
Usually, it solved by looking/modifying directly at form (*.lfm / *.dfm) using text editor such notepad.exe
Sometime, it just solved by Cut & Paste (Ctrl+X, Ctrl+V) the old component, in place. This step usually will "refresh" the component's stream in form's stream (*.lfm)
if these don't correct, I try all the step in new computer or new VirtualMachine.
if the last step works, I know that I need to cleanup the temporary files by Restart computer,
Alternative step that I could pass is:
install new instance of Lazarus. Checkout a clean SVN (or Clone a new GIT).
That's all I usually did to solve my "non-logical problem".
I hope it helps.