It's definitely not normal, but it may not be a real problem.
As SDG says, pressure relief valves could be plauing a part in this, but they shouldn't end up completely deflating the tubes.
As the tube should have several independent sections it is surprising that the tube goes doen completely. It's not unheard of for valves to leak, but I wouldn't expect them all
to. Another possibility is that the tube fabric may have become slightly porous, but I have only seen this on very old inflatables (like 20 years old). Leaking seams or multiple very small punctures are also possible, but not very likely.
How old is the boat? What sort of valves are fitted? Have you tried spraying it with soapy water to see if yoiu can find any trace of air leaking?
In the meantime, just pump it up each time you use it and don't worry too much!