I can confirm this bug
I also tried in VM
(Windows XP Service Pack 3 (5.1.2600), Windows Live Messenger 14.0.0416) and upgraded Plus! from 4.90.0.392 to 5.00.0.691 and nicely detected latest version (5.01.0.706) though It doesn't detect 5.01.0.706 from 5.00.0.702
Windows 7 Ultimate x64 Service Pack 1 (6.1.7601)
Windows Live Messenger 15.4.1109
Messenger Plus! 5.00.0.702