Smart Quotes Delete Initial Capital Letter. Help, please.

Mac Mini, Big Sur 11.7.10.

This problem occurs regardless of whether or not the Smart Quotes checkbox is checked, and is most noticeable and problematic in MAIL. I type a quotation. I type the quotation mark and the first word, which begins with a capital letter. When I finish the word and press the space bar, the system goes back, deletes the capital, and replaces it with a space. I try to type "Real, and end up with " eal. Cannot find any info on this; suggestions welcome. Thank you.