Physical (from weapons and skill) gets converted to Fire by weapons.
Transmuter has 100% Physical to Acid so I think as a result 50% of Phy → Acid and 50% of Phy → Fire.
It’s a loss because you have little %Fire and little Fire resist debuff and a reason why we usually focus in GD on 1 damage type.
This flat Fire and Burn on your weapons could be somewhat useful if converted to Acid (i.e. by global conversion from Path of the Three) but they are converted fully to Vitality by the transmuter which has priority.
I think you should switch to some Acid weapons, not sure what are the best (target farmable) ones for this build,
you could check some guides. For example these
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the ones from Stupid Dragon’s guide that was mentioned.
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Rift Scourge Slicer has Cold → Acid in 1.1.9.2 patch (Public Testing FAQ)
which you could use to convert Cold from Lethal Assult and base Total speed bonus
btw you could upload your current setup into GrimTools.com so that everyone can have a look at it