The use of "left Flight Level.." seemed perfectly clear to me over the past 35 years. And I don't recall any pilot problems either.
There was a certain group of controllers who were concerned about ATC issuing a clearance to "maintain" an intermediate altitude or level. EG "Maintain one thousand five hundred, cleared for take off." The protagonists suggested that a more correct phrase would be "Climb to and maintain.." There was never enough support to change the AIP (and I don't think it was necessary).
You can drive yourself nuts trying to come up with completely unambiguous phrases. Many can be misinterpreted. And this includes the many USA and UK peculiarities.
This reminds me of a funny episode at a GA airport when a pilot did not read back his cleared altitude when departing on an amended clearance. The aircraft became airborne and the controller tried once more in slow clear words; "ABC read back altitude"
The pilot replied "ABC, Back altitude one thousand!"
(Actually you had to be there - it doesn't sound quite so funny now!)