I think it is something to be avoided. There is no provision for bonding in an LB, it is difficult to maintain 150mm of conductors for splicing, and some may consider an LB to be a raceway, and we know joints are not permitted within raceways.That being said I dont think there is a rule that specifically prohibits splicing in an LB.
(2) With Splices, Taps, or Devices. Only those conduit bodies that are durably and legibly marked by the manufacturer with their volume shall be permitted to contain splices, taps, or devices. The maximum number of conductors shall be calculated in accordance with 314.16(B). Conduit bodies shall be supported in a rigid and secure manner.
IMHO, splicing within an LB, LL, LR, C, T, etc. is not a respected trade practice, and not often seen.