In the Custom Dictionary dialog, the list included two items: 1: RoamingCustom. This tip did reset whatever needed resetting. After adding a word to the two custom dictionaries, spell check still trips over the term every time it appears.
Then remove the checkbox next to the dictionary item, then check it again.
Note: This option is also available in other Office applications such as Outlook, Excel, Publisher, etc. You can change this by adding custom words to the dictionary. Does Microsoft Word regularly detect words that you commonly use as misspelled words? Disable, Enable, and Set the Default Custom Dictionary in PowerPoint 2016 for Windows
