Does aluminum oxide react with rubidium?
As suggested in the comments, rubidium aluminates do exist.
Aluminates such as $\ce{RbAlO2}$ and $\ce{Rb6Al2O6}$ are prepared at higher temperatures (above 550 °C [1]) or even from the melt.

However, alkali metal vapors at elevated temperatures act as reducing agent, so I wouldn't expect aluminates anyway.
On the other hand, aluminides such as $\ce{RbAl}$ (Zintl phase) are formed from the melt at much higher temperatures (above melting point of aluminium) and pressures exceeding atmospheric.

Since the corundum-coated plate is supposed to perform in vacuum under mild temperatures, I think it shouldn't be affected by rubidium vapors.
There is a possibility of coating embrittlement over time as rubidium diffusion occurs, but I wouldn't expect it to be an issue.

References


Schläger, M.; Hoppe, R. Darstellung und Kristallstruktur von $\ce{K6[Al2O6]}$ und $\ce{Rb6[Al2O6]}$. Zeitschrift für anorganische und allgemeine Chemie 1994, 620 (5), 882–887. https://doi.org/10.1002/zaac.19946200522.