Check the Monzo app notifications too. I’ve had specific toggles de-toggle for unknown reasons before. The following guide may differ depending on the Android version and Launcher used, but it may help:
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Long press on the Monzo app icon and tap on ‘App info’
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Tap on ‘Notifications’
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Ensure all toggles - or the specific ones you actually want - are enabled (I believe the ‘Important’ toggle is the one for approvals):
Boy, do I wish the in-app counts of notifications for credits and debits were reversed…
If this doesn’t work, the usual log-out of 
, uninstall the app, reboot the device, download & install the app & login again should clear any such ‘funnies’