Lambano is one of those #CLASSICALGREEK verbs whose variety of forms confuses some students. Yes it seems odd at first that a *LAB stem with a SHORT vowel changes to LEEB however other verbs do this too!
If you have access to a copy check section 611 in Goodwins Greek Grammar.
Lambano is not the only Greek Verb that does this!
As for the stem changes B > Ph is quite logical if you remember Greek has aspirate P not and NO V and an ingrained avoidance of triple consonant clusters so its Lambano becomes LEEpsomai leep-so-mai or lee-pso-mai.
Don't think of it as irregular but quite regular if you understand the patterns of Greek phonology.
Aspirated consonants NOT fricatives and consonants plus a S for future.
So Lambano Future leepsomai Aorist elabon Perfect eileepha