Hello Nadav
This is an excellent question. According to RFC 5375 in section B.2.4 EUI-64 ‘u’ and ‘g’ Bits, it states the following:
When using subnet prefix lengths other than /64, the interface
identifier cannot be in Modified EUI-64 format as required by
[RFC4291]. However, nodes not aware that a prefix length other than
/64 is used might still think it’s an EUI-64; therefore, it’s prudent
to take into account the following points when setting the bits.
The document continues to state several considerations that should be checked carefully. In general, depending primarily on the vendor, using a prefix other than /64 may be possible in conjunction with EUI-64 IPv6 address computation, however, because it may have unpredictable results, it’s a good idea not to implement it.
I hope this has been helpful!
Laz