Boardman Pro Carbon SLR - Crankset Problem

Morning Everyone,

I hope everyone is well and any help on this topic will be much appreciated.

I purchased a second hand carbon road bike last year after my original bike was pinched a week before I underwent a charity cycle up Aple d'Huez and have had a few problems with the crankset.

I was cycling uphill and felt a strange sensation ending with the LH crank arm coming loose having stripped the lower quarter of the spline teeth.

I have noticed that the bike is fitted with a 73 mm overall length PF30 bottom bracket with a FSA PF30 24 Reducer and Ultegra FC R8000 crankset.

My research tells me that a 68 mm bottom bracket is needed for all road setups and im wondering if this has caused the problem.

My questions are; how can this be remedied as only one thickness reducer is on the market and why was this road bike was manufactured with a 73 mm bottom bracket seemingly incompatible with road groupsets.

Regards,

Rob.