Nop 4.3 FedEx Plugin Not Working With Standard LTL Freight

We recently upgraded from Nop 3.9 to 4.3 and are using the current FedEx V 1.36 plugin. Everything is working properly with the exception of FedEx Standard LTL Freight.  We did not encounter this issue with Nop 3.9

If I select FedEx 1Day Freight, FedEx 2Day Freight or FedEx 3 Day Freight as the Carrier Services, the plugin returns the expected astronomically high shipping rates in the shopping cart.

However, if I select FedEx LTL or FedEx National Freight Carrier Services, the shopping cart displays the error "Shipping options could not be loaded".

The log file is of little use as the FedEx plugin posts no errors to the log.

At this point, I think I need some technical assistance to further troubleshoot the issue and modify the FedEx plugin to properly quote FedEx standard LTL rates for FedEx Freight Priority
FedEx Freight Economy services

Selecting services in the FedEx plugin configuration page acts as a filter, so that when the response is received by the plugin (from FedEx API), the plugin only shows the selected services, even though others may be returned.    If you're not seeing your selected services, then it probably means that the FedEx API is not returning them.   It's possible that some change to the plugin causes it not to make the proper request as it did in the past (e.g. maybe a new version of the FedEx API is being used), but it could also be a problem on the FedEx end.  I suggest you contact FedEx technical support, and ask them why the n-Day services are returned rather than the others.   (If the do say they are returning them, then there is likely a bug in the plugin ;)