Seems commonly accepted that higher temps create more diacetyl, but OTOH it also helps the yeast reduce it more quickly. Some examples:
1) White Labs.
link "Fermentation parameters that effect diacetyl levels are temperature and aeration. High fermentation temperatures promote diacetyl production, and low aeration levels when yeast is pitched will produce less healthy yeast, which are prone to higher diacetyl production."
2) Fermentis.
link "Using higher temperatures for your brew will increase the ester and diacetyl formation."
3) BYO mag.
link . "Temperature. Higher temperatures lead to more diacetyl formation. For example, a lager beer that is fermented at 57° F can produce up to three times as much diacetyl as a similar beer fermented at 40° F."
4) Study in Journal of the Institute of Brewing.
link . See Figure 6. Also: "Firstly, since diacetyl production and consumption rates increase with temperature; as well as the fact that the final diacetyl concentrations are quite similar in all the cases examined,... these small variations may introduce major differences in the flavour of the beer."
5) Article in Institute of Brewing, "Diacetyl and its control...".
link . "Increased fermentation temperatures lead to higher initial diacetyl production rates as a consequence of increased yeast growth, but also produce more yeast mass to reduce the diacetyl to 2,3-butanediol and increase the reaction rate of the oxidative decarboxylation of α-acetolactate to diacetyl..."