VBOX modifies base VDI file after snapshot is made ???

[froff]

hello
I had a plan to use VM with big base VDI file frozen as snapshot and a small incremental one. Such configuration reduces incremental backup size because main big VDI remains unchanged.
But I noticed that modification date in base VDI changes! So my strategy seems to be useless. Base VDI changes even if there is more independent "branches" from it. If there are more
snapshots in sequence (for example base snapshot -> snapshot 1 -> snapshot 2 ->current) all VDI-s on the patch changes when machine is turned on.
Is it normal? Is it any way to solve this problem?
Today I'm going to compare two versions of snapshot modified in this way and maybe block write permission to snapshot VDI in order to check if those changes are meaningful.
Any other suggestions?

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new information:
After some investigation I recognized that starting VM does not cause modification of snapshot files. Modification dates of all snapshot VDI files changes only when last (current) VDI is compacted.
Fortunately it seems that only date is changing and file remains exactly the same - I'm not sure if it is true in all situations but probably yes.

[mpack]

The base VDI is frozen, but not write protected. It will remain unmodified in all important respects unless you apply a tool directly to it (e.g. VBoxManage). If you are saying that something else happens then you need to be explicit about your evidence.