The Roomba-500 charger output is 22.5V, NOM. for load current between zero amperes and 1.25A. When current draw exceeds 1.25A (as it does in the initial Hi-Rate phase) Roomba's charging PSU will lower its voltage to prevent excessive current being drawn. Not only is this a safe tactic for the PSU and on-board charging control components, but it is part of Roomba's charging control scheme!
If your PSU's stated "750mA" falls below 400mA for 30-minutes Roomba will announce "err5" and halt the charging process.
It could be educational for you to measure (via use of a DVM) two parameters resulting while applying your "24V, 0.75A" PSU to Roomba. The values to measure are: "charging voltage across battery terminals" and "actual charging current".
Doing those measurements would require you to access Roomba's main_PCA (upper surface) so voltmeter probes can contact four specific PCB pads on the PCA.
An alternate path, easier to do but less demonstrative, would be to resistively overload your Universal PSU so it delivers current in excess of its rated 0.75A (say, to load it enough to make its output voltage fall so low that resultant load current drops below 0.4A). This test assumes the PSU's voltage / current regulator uses a "current fold-back" circuit -- a different protective function than iRobot
uses (voltage roll-off protection).