[RPG] What happens if you no longer meet the Strength requirement for your armor

PHB Chapter 7 says this about hit points under the "Constitution" subheading (p. 177):

If your Constitution modifier changes, your hit point maximum changes as well, as though you had the new modifier from 1st level. For example, [...] if you are 7th level and some effect lowers your Constitution score so as to reduce your Constitution modifier by 1, your hit point maximum is reduced by 7.

When your Con modifier somehow decreases by X, your Max Hit Points are decreased by an amount equal to X*Level.

What happens to HP when Constitution decreases? / Are the HP lost when a character is no longer attuned to the Amulet ?

If your Con modifier decreases, your Max Hit Points decrease; any current HP over your new Max HP are lost, too.

Belt of Giant Strength

While wearing this belt, your Strength score changes to a score granted by the belt. If your Strength is already equal to or greater than the belt's score, the item has no effect on you

(emphasis added)

Manual of Gainful Exercise

If you spend 48 hours over a period of 6 days or fewer studying the book's contents and practicing its guidelines, your Strength score increases by 2, as does your maximum for that score

Combining the items

Looking at the difference in wording, it's clear that the Manual provides a permanent bonus to your strength - there are no conditions to it. So your strength score would change to a 10, with the maximum changed to 22.

The belt has a condition (you have to be wearing it), and when it is met your score changes to 29. It's not a bonus (e.g. +17), it's a replacement. Whatever your strength score beforehand, if your strength is less than 29 it is becomes 29. The strength gain from the manual still doesn't bring you above 29, so it wouldn't change the belt's effect.