Dear statalist members,

I am using ppmlhdfe to investigate the bilateral trade flow and have a question about handling singleton issue.

Here is my code:
ppmlhdfe import lndistance lntariff contiguity common_language agree_cu, absorb(imptime exptime, savefe) cluster(impexp) nolog d

- imptime: importer-year pair fixed effect
- exptime: exporter-year pair fixed effect
- impexp: importer-exporter country pair

Specifically, my data has 189 importer countries and 211 exporter countries from 1989 to 2019, yielding 905,138 of total observation number.
However, after the estimation, only 95,827 observations remained.
The stata says that it dropped 809,303 observations \that are either singletons or separated by a fixed effect.
My data set has so many zero values for dependent variable, so I am guessing that it caused so many singletons.

With an option, "keepsingleton", I could keep almost all the observation numbers with no changes in the estimated coefficients.
Stata says that "ReLU method dropped 8 separated observations in 1 iterations
Converged in 17 iterations and 246 HDFE sub-iterations (tol = 1.0e-08)"

Here are my specific questions:
1. Is there anything wrong in my code?
2. Is my ppml estimation with so many singleton meaningful?
3. Can I keep use the "keepsingleton" option for my estimation?