When we follow the hashtag easy-installer which I added to your OP we see a run of similar reports for the Samsung S7, and other.
S7
S8
- Galaxy S8 does not reboot into e Recovery screen – this thread seems to contain quite a useful work through of the problem, and leads to [1].
S9
What sticks in my mind is that the e-recovery does not seem to be able to handle being flashed to a “new device”. I think especially the newer threads had this issue but it does not apply to all the threads I listed.
[1] Galaxy S8 does not reboot into e Recovery screen - #21 by Xxpsilon.