I have $19-20,000 of deducted medical expenses, every year, so I do itemize.
You need to get an explanation from your tax professional then, because you are under the wrong impression. There is not a $300 limit on charitable contributions for people who itemize.
This is Form 8283 for 2020:
https://www.irs.gov/pub/irs-pdf/f8283.pdf
These are the instructions:
https://www.irs.gov/pub/irs-pdf/i8283.pdf
From the instructions, “You must file one or more Forms 8283 if the amount of your deduction for each noncash contribution is more than $500. You must also file Form 8283 if you have a group of similar items for which a total deduction of over $500 is claimed”
Despite the typical IRS jargon; it is not really very complicated. From the description of the problem, the second page of form 8283 did not get included in the electronic filing and has been sent in separately twice now in paper form. The IRS is hopelessly behind in their paper communications and it is causing a lot of stress for filers who have received notices and then no acknowledgment when they do what is asked and in fact receiving more notices.
Sorry I accidentally pasted the final paragraph that does not apply to you in my response.