So I followed the tutorial here, and the bag file is also from this tutorial: bagfile. What I did is just follow the tutorial, and in the Rviz the map only shows the first scan result, with

rosrun map_server map_saver 

saving the map, and the result is the same - only the first scan result.

and the terminal which runs the command:

rosrun gmapping slam_gmapping scan:=base_scan 

shows following warning:

[ INFO] [1557266797.433063738, 124.693075077]: Laser is mounted upwards.
 -maxUrange 29.99 -maxUrange 29.99 -sigma     0.05 -kernelSize 1 -lstep 0.05 -lobsGain 3 -astep 0.05
 -srr 0.1 -srt 0.2 -str 0.1 -stt 0.2
 -linearUpdate 1 -angularUpdate 0.5 -resampleThreshold 0.5
 -xmin -100 -xmax 100 -ymin -100 -ymax 100 -delta 0.05 -particles 30

[ INFO] [1557266797.461074088, 124.713230408]: Initialization complete
update frame 0
update ld=0 ad=0
Laser Pose= -19.396 -8.33452 -1.67552
m_count 0
Registering First Scan

[ WARN] [1557266797.918746187, 125.178359316]: Detected jump back in time of 0.00825748s. Clearing TF buffer.

[ WARN] [1557266798.363414013, 125.622761560]: Detected jump back in time of 0.00749887s. Clearing TF buffer.

[ WARN] [1557266799.211517500, 126.471356075]: Detected jump back in time of 0.00677431s. Clearing TF buffer.

[ WARN] [1557266799.717561975, 126.976918037]: Detected jump back in time of 0.00535936s. Clearing TF 
buffer.

[ WARN] [1557266799.969104666, 127.229202756]: Detected jump back in time of 0.00531196s. Clearing TF buffer.