See my responses below
Did this
always happen or just started, or after the tankless was installed?
--Unsure if it happened before tankless was installed (in Sept 2019).
Are you on a well or city water?
--On city water
Sounds like a water hammer effect and on the recoil the pressure drops just enough to activate the tankless. If you are outside when a zone shuts down you hear a thump?
--I do hear a thump
Is it any zone when it shuts down?
--any of the 5 zones causes it
If you are on city water and the irrigation system was installed when the home was built, it had to pass inspection with a anti backflow or anti siphon device.
--there is a Richdel 709PR (Irritrol) 1" shut off valve with anti-siphon installed now
Do you have a indexing valve for the zones or individual valves for each zone? With an indexing valve when the timer shuts off the solenoid so the it can rotate to the next zone, you may see all sprinklers momentarily squirt water. That can drop the pressure for a second or two.
--individual solenoid per zone
Not sure what the fix can be but it will depend on the cause. You can put a pressure gauge on a spigot that is before the irrigation system and look at the needle as a zone shuts off. You might be able to get an idea on the pressure drop.
Is there an adjustment on the tankless?
-- no adjustment on the tankless
Some irrigation systems may have a shut off valve before the backflow preventer. Close it down to reduce the water flow to the irrigation system. When a zone shuts down it should reduce a hammer effect if it is the cause.
-- I did reduce the flow to less than 1/2 normal flow but issue still happened
Then install a hammer arrestor. This one (the HD link) can be threaded in a spigot and get a cap of the other end before cutting into the system whereby the arrestor doesn't take care of the problem.
With the pressure gauge it will give you what your normal static pressure is and when the water flows to the irrigation system. If the pressure is over 60 PSI while the water is flowing, a pressure reducing valve maybe needed.
https://www.homedepot.com/p/The-Plu...estor-for-Washing-Machine-HA34WM-NL/308601314
https://www.homedepot.com/p/Watts-3-4-in-Plastic-Water-
