Is it true that the 400,000 baht for a marriage visa needs to remain untouched in the bank for 2-3 months after the visa is issued?

No that is not true, some immigration offices will require the 400,000 baht to remain in your account during the under consideration period, but after the stamp is in your passport you are free to take it all out. Just make sure it is back in your account 2 months prior to your next extension.

The only visa/extension that requires 400,000 is the marriage to Thai visa.

If you apply for the initial 90 day visa, you'll need to keep it in your bank account because you'll need to be able to show that it was in your account for 2 months when you apply for the 1-year extension after 60 days.

But after you get a 1-year extension there is no requirement for the month to stay in your account. You'll need to get it back up to 400,000 a full 2 months before you apply for the extension the following year.

It's the retirement visa that requires you keep it at 800,000 baht for 3 months after you receive the extension and then the account can never drop below 400,000 not even for a day.