Map tier

At the map device, I have selected "Cold River, Tier 6" destination, and have put into the device a Tier 16 "map" ("ticket").
Then, when travelled, (unexpectedly) I noticed that the tier6 ColdRiver suddenly became a tier16 ColdRiver ("Monster Level: 83").
Is it a bug? If not, please explain.

A question arises: without any Voidstone, and having only 1 tier16 "ticket", is it possible to "farm" tier16 maps, infinitely (sustainably)? If 1 tier16 ticket is not enough: how many is enough?

This is not a bug. Maps are no longer named (old ones in your stash still have their old names but they are irrelevent), instead the map name is determined by what is on the Atlas.

Cold River at T6 on the atlas can be completed by using any T6 map, or a higher tier which will result in the map being that higher tier.

Yes, you can use T16 maps on any map node on the Atlas now :)