The rod gives +X to all warlock spells and the Tome pact gives you spells that are treated as Warlock Spells. However, DDB's char sheet doesn't detect this and the buff only applies to spells learned directly from the class' spell list.
To Replicate:
1) make a Warlock of Level 3 + and pick at least 1 spell that needs a save DC/Attack Roll
2) choose Pact of the Tome and pick some spells that require a save DC/Attack Roll
3) attune and equip a Rod of the Pact Keeper
4) compare in the spells section of your completed warlock's char sheet.
The rod gives +X to all warlock spells and the Tome pact gives you spells that are treated as Warlock Spells. However, DDB's char sheet doesn't detect this and the buff only applies to spells learned directly from the class' spell list.
To Replicate:
1) make a Warlock of Level 3 + and pick at least 1 spell that needs a save DC/Attack Roll
2) choose Pact of the Tome and pick some spells that require a save DC/Attack Roll
3) attune and equip a Rod of the Pact Keeper
4) compare in the spells section of your completed warlock's char sheet.
This is a known issue with the Pact of the Tome cantrips.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)