QGIS shows min/max values at 2%/98% quantiles by default, and replacing nodata by zero(0), or adding zeros to the dataset, can shift its range.
Let me try to illustrate what (i think) had happened, using a sample dataset taken from USGS Lake Tahoe Basin.
Layers Panel shows 2 layers,
Reclassified Grid which came from SAGA "reclassify values" tool which replaced NoData values of
tahoedems (original DEM) by zero (0).
We notice its maximum value also changed from 9327.77 to 9275.77.
It is confirmed by Min (0) and Max (9275.77) values on the
Layer properties | Style. (see below image)
----- Min and Max values (default display) of Reclassifed raster ----
However, what we are looking at the above image is
Cumulative count cut (2%~98%).
Changing the setting to
Accuracy | Actual (slower) gives us real data range. (0 ~ 10890, as below)
----- Min and Max values (actual range) of Reclassifed raster ----
This maximum value of the original raster (tahoedems), which range 4616~10890 ft confirms that maximum (10890) remains unchanged.
----- Min and Max values (actual range) of original DEM ----
The distribution of cell values, which was 4616-10890 originally, was skewed toward lower-side by adding 0-values where there were no data (in other words there were NoData cells). This shifted 2%/98% cumulative count, so it looked as if it changed maximum value, too.