It has happened to my roadster too. But it only happened when I did not put any effort into the change - thinking the dog gears were not fully engaged.
Yep, the not locking into second is s far different feel than when it slips in second. Thinking those are just the missed shifts which are pretty common.
Triumph only had the engine for a few days, went and had a look and spoke to the tech doing the job, he has done quite a few.
They were very helpful with any questions I had.
Got it back and refitted and fired up in 7-1/2 hours., pretty happy
All I used was a front wheel chock and pivoted the rear up with an engine crane. Raised the engine up on the ATV lift and lowered the frame down in a arc onto the motor.