Does Apple Pay Still Work in Lost Mode?
This is intended behavior when the device doesn’t respond to Apple Push Notifications to confirm locking and deactivation of the cards. It could be out of battery or off network or just unable to process the erasure of the cards. 

Normally when there is a good cellular data connection setting your iPhone to Lost Mode both disables Apple Pay and suspends and confirms deactivation Apple Pay cards.

In your situation, provided you received an email confirming your iPhone is in Lost Mode, Apple Pay will be disabled and unusable on this device until Lost Mode is turned off.

Entering your device password on the device will turn off Lost Mode and re-enable Apple Pay, but won't un-suspend suspended cards. Normally you have to un-suspend the cards by entering your Apple ID username and Apple ID password. Since your cards were not suspended, they may be usable after entering only your device password.

If you have a weak password or someone else knows your password, then you may want to contact your card provider to suspend these cards.