There's no limit on L permit duration. I'm in my 4th year on an EU-L permit, even with a 12 month contract. I haven't had any issue with being on EU-L though, and as far as I can tell rights are the same as EU-B except the B often lasts longer & so will allow for a longer time to stay (> 6 months) if you become unemployed. I believe EU-L can buy property same as an EU-B.
Maybe you should try convincing Migros that EU-L permits aren't what they used to be and that there is no 1 year limit.