Mustard Flavor disappears

The flavour of water-based mustard reaches its peak about 15 minutes after you make it. After that it declines quite rapidly. If you're making fresh, then you really should make just enough for 'today'.
The predominant 'hit' of mustard is very volatile & will quickly dissipate.

Store-bought mustard is vinegar- & oil-based, which makes the flavour last longer, but at a loss to the overall mustard hit.
It shouldn't degrade in anything like the short time water-based does… it will be as bland tomorrow as it was today ;)