Payplan Joomla User Type App Gives Fatal Error

I have 5 Joomla User Type Apps set-up in my Payplan component. Out of these, when I open the main Joomla User Type App instance, the one which has a lot of users linked to it, it throws the below error:

Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 31277716 bytes) in /SITE/libraries/vendor/joomla/string/src/phputf8/mbstring/core.php on line 114

I have PHP `memory_limit` set to `128M` which should be more than enough. Can you tell me how this issue can be resolved? The App seems to work and users are getting added to the respective user group. But I am unable to open the App Instance and the above error shows when I go to: Components -> Payplan -> Apps -> App Instances -> Main Joomla User Type

I am using:

Joomla 3.8.1
Payplan Version : 3.6 | Build :3
Joomla User Type Version : 3.4.0