Actually, based on @jlin's comment "It seems like events are getting deleted somehow" ... the datagen by default starts the log file with data from 1 year ago.
Can you check your default/system value for "frozenTimePeriodInSec"?
if it's changed from the default out of the box value, to something lower than 1 year, your data will be frozen as you index, which could account for the decrementing % indexed value.
splunkd.log should tell you if data is getting frozen
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