DHCP client cache server route

We had an issue with implementation of AD with one PC
that kept getting IP address subnet range from old
existing DHCP server. The topology is as follows:

Floor 2 is the implementation floor for new DHCP with a
switch connected to a router going out through corp WAN
to HQ through Telus (New).

Floor 1 has the same setup but going through another ISP
vendor, Bell. This was the original and still existing
connection for DHCP.

The switch on floor one and two are connect on port 24.

5 new PCs were rolled out and received DHCP through AD on
the second floor with no issues. There was an existing PC
on the same floor that also needed to be connected to the
network to receive DHCP, but it kept on retrieving the
old subnet range from the original DHCP. In order to
mitigate this issue, we broke the link between the two
switches.

I suspect the issue is due to DHCP gateway being cached
by the one PC that was originally on the old network.
Does anyone have a link to justify this and a solution to
clear this cache issue for future implementation? Any
help would be appreciated.